Display devices in use today typically employ liquid crystal displays (LCDs) and more recently, organic light emitting devices (OLEDs). FIGS. 1a and 1b illustrate examples of display devices such as active and passive matrix display devices and their operations briefly.
Matrix displays typically contain a grid of small picture elements (pixels), which can be switched to form characters and display graphics and video images. The electrodes are patterned as a series of stripes, with the stripes 11, 12 on one glass piece running perpendicular to the stripes on the other glass piece. The electrodes are made from a transparent, conductive material, usually indium-tin oxide (ITO). Switching cells or pixels 10 are formed were the stripes overlap as shown in FIG. 1a. In liquid crystal displays, the pixels are comprised of liquid crystal material sandwiched between the electrodes. In OLEDs a series of layers of organic semiconductor material, one of which emits light on application of current, are sandwiched between the electrodes.
Passive matrix displays use a simple grid to supply the charge to a particular pixel. That is, the rows or columns are connected to integrated circuits that control when a particular column or row is biased with the proper display drive voltage. To turn on a pixel, the integrated circuit biases the correct column and the correct row with the drive voltage signals. The row and column intersect at the designated pixel, and the row and column bias voltage result in the correct voltage at that pixel.
In active matrix displays a drive scheme is used that employs a storage capacitor 14 and it transistor switch 13 at each pixel site as shown in FIG. 1b. Active matrix displays most commonly use thin film transistors (TFT). The TFTs, usually microscopic in size are arranged in a matrix on a glass substrate and connected to the row and column busses as shown in FIG. 1b. To address a particular pixel, the proper row is biased switching on the TFT gates connected to that row. Then the correct column is biased with the proper drive voltage. Since all of the other rows that the column intersects are turned off, only the storage capacitor at the designated pixel receives a charge. The storage capacitor 14 is able to store electrical charge and hold the bias voltage on the pixel 15 after the TFT gate is switched off and until the next refresh cycle. This means that the signal does not have to be refreshed as often and thus larger matrixed arrays are possible. In addition, the transistor prevents crosstalk by only switching on the pixel when the frill switching voltage is applied.
The display devices including the above described matrix display devices, however, have problems associated with them such as poor viewing characteristics, for example, in high ambient illumination environments, poor visibility over vide viewing angles, and/or high power consumption. Accordingly, there is a need for more efficient display devices.